The challenge begins...

As the days progress, zombies will get stronger, faster, and will deal more damage. Past day 100, the scaling will eventually stop, but at this point they will be so powerful that an average gun and some iron armor will mean nothing. To prepare for this, you should take a trip to the Void, a dimension below the overworld, accessible at 0,-64,0. You should bring a lot of weapons, your best armor, and plenty of food. 

The Void is a dangerous place, full of many normal zombies, and also more powerful mutant versions.

 In this dimension you should mine for precious ores including Lavendite, Onyx, and Bismuth. Bismuth, in conjunction with Netherite found rarely in the overworld, is used to make the best armor in the game. Lavendite and Onyx (and some Bismuth), will need to be used to make advanced Timeless and Classic guns which deal more damage overall to enemies. 

To leave the Void, simply pillar upwards until you get teleported back to the Overworld.

Back in the Overworld:

To make the creation and collection of valuable resources easier you should invest in the creation of Create machines. These devices can harness energy (such as through water wheels), to power machinery to smelt, cut, and harvest items. This can eventually lead to the complete automation of some resources. This will be critical in the late game, as late game defenses and tools cost a lot.

To better traverse the land you can also create trains through the Create mod, or planes and other flying devices (note: these do not work in the Void dimension).

Netherite is not automatable, so you will want to go mining for this valuable resource to create Bismuth-Netherite armor. 

As the zombies get stronger, level up your home base with walls and traps and stock up valuable resources for use in the future.